A continuous alloying treatment furnace for a hot dip zinc coated steel sheet is provided above a zinc pot 2 which applies galvanization to a steel sheet 1, as shown in FIG. 2. That is, above the zinc pot 2, a wiping nozzle 3, a heating furnace 4, a holding furnace 5, and a cooling zone 6 are disposed upward in this order. The steel sheet 1, passing through the zinc pot 2, has its surfaces coated with zinc. After the steel sheet 1 is controlled by the wiping nozzle 3 to have a predetermined weight of coating, it is passed through the alloying treatment furnace comprising the heating furnace 4, holding furnace 5, and cooling zone 6. During this process, alloying of the coated layer is performed.
Such a galvanized steel sheet, which has been subjected to alloying treatment, is better in weldability, workability, paintability, and corrosion resistance than an ordinary galvanized steel sheet. Thus, it is used frequently as a steel sheet for household electrical appliances and automobiles.
Alloying treatment of the zinc coating needs to be performed to obtain an iron-zinc alloy layer composition which is ideal, particularly, for ensuring both coating adhesion and press formability at the same time. Coating adhesion and press formability are important quality factors, because the former characteristic prevents powdery peeling of the coated layer, called powdering, during working, while the latter results in an alloy layer with a low sliding resistance, thereby reducing a load during forming. Specifically, the surface of the steel sheet after alloying treatment should have a coating composition consisting mainly of a .delta..sub.1 phase while minimizing a .zeta. phase with a high sliding resistance, and a hard, brittle .GAMMA. phase which deteriorates powdering resistance, as shown in FIG. 3.
The constitution of the alloy layer is determined by a heat cycle of heating, holding, and cooling, which have to fulfill the following requirements as shown in FIG. 4:
(1) Heating: Rapid heating for suppressing the .zeta. phase. PA1 (2) Holding: Control of the holding temperature and holding time such that the minimum temperature is T.sub.1 or higher, and the holding time is t.sub.1 or longer, for suppression of the .zeta. phase, and that the maximum temperature is T.sub.2 or lower, and the holding time is t.sub.2 or shorter, for suppression of the .GAMMA. phase. PA1 (3) Cooling: Rapid cooling for suppressing the .zeta. phase. PA1 1) Involving a method which measures impedance on a load side as viewed from a high frequency power source to calculate the resistivity of a material to be heated, and which, based on the values of the relevant properties, detects the accurate temperature of the material to be heated. PA1 2) Involving a heating temperature control method which measures the impedance on the load side as viewed from the high frequency power source to calculate the resistivity of the material to be heated, thereby making it possible, based on the values of the relevant properties, to detect the accurate temperature of the material to be heated, and to obtain an appropriate temperature.
It is well known that induction heating is suitable as means of obtaining rapid heating and a highly accurate heating temperature (=holding temperature) among the above requirements. Various induction heaters for alloying have been proposed (e.g., Japanese Unexamined Patent Publication Nos. 294091/92, 228528/92 and 320852/93). Thus, an induction heating type heating furnace is used as one of means for obtaining a heat cycle for forming an alloy layer of a galvanized steel sheet.
FIG. 5 shows an example of circuit configuration of an induction heating apparatus.
A material 8 to be heated is passed through a solenoid coil 7, and a high frequency current of a frequency from several kHz to 100 kHz is applied to the solenoid coil to flow eddy currents into the material 8, thereby heating the material 8. The generated heat distribution and the temperature distribution, in the width direction, of the material 8 to be heated by induction heating vary with the type and width of the material 8 as well as the frequency of induction heating. The oscillation frequency of the source of induction heating is nearly in synchronism with the frequency of the heating coil and the capacitor as a resonance circuit. Thus, the frequency of the high frequency current flowing in the heating coil is determined by the capacity of the resonating capacitor and the inductance of the solenoid coil. The inductance of the solenoid coil is determined by its shape and number of turns.
On the other hand, the appropriate holding temperature T.sub.2 -T.sub.1 and the appropriate holding time t.sub.2 -t.sub.1 vary with the weight of coating, and also vary with the type of steel of the steel sheet. To obtain the amount of heating, the heating temperature, and the holding temperature that are highly accurate, it is necessary to measure the temperature of the steel sheet with high accuracy.
With the conventional alloying apparatus, a temperature gauge 9 is mounted between the heating furnace 4 and the holding furnace 5, as shown in FIG. 2, to measure the temperature of the steel sheet that has passed through the heating furnace. This is intended to obtain appropriate temperature conditions. Based on the measured temperature of the steel sheet, the amount of heating is adjusted so that an appropriate temperature will be obtained. Furthermore, the alloy layer of the steel sheet on the exit side of the cooling zone 6 shown in FIG. 2 is measured with an alloying degree meter 10. Based on the results of measurement, the amount of heating is adjusted so that an appropriate alloy layer will be obtained.
To measure the temperature of a steel sheet, the conventional alloying apparatus used a radiation thermometer or the like as the temperature gauge 9 of FIG. 2. However, a radiation thermometer cannot measure the temperature accurately, because, at the exit side of the heating (alloying) furnace 4, the surface of the steel sheet is white to silver in color, and not black. This makes it difficult to obtain a highly accurate heating temperature, posing difficulty in forming an appropriate alloy layer. The state of alloying is measured with the alloying degree meter 10 after cooling. However, the alloying degree meter 10 is apart from the heating furnace 4, so that a delay occurs even upon feedback control. As a result, a galvanized steel sheet may be produced at an inappropriate heating temperature. With a zinc coating alloying apparatus, therefore, it is necessary to measure the temperature at the heating furnace 4, and control the amount of heating based on the measured temperature.